Abstract

The problem of the
motion of boundaries (bow shock and magnetopause) was
studied using several nearly simultaneous crossings
of the bow shocks and/or magnetopause
identified by plasma and magnetic field
measurements onboard Interball 1 and
Geotail satellites. One of such
observations, on 11 October 1996, when the satellites
were at a distance of up to
30 RE from each other, shows two different
events: simultaneous
bow shock and magnetopause
sunward motion as a response to solar wind plasma and
IMF disturbances and almost simultaneous bow shock
sunward and magnetopause earthward
motions. Some causes of such behavior of the boundaries,
including the influence of
hot flow anomalies of the solar wind, are discussed.

1. Introduction

To study the dynamics
of solar-terrestrial relations, it is rather important to know
the instant magnetospheric response to solar wind disturbances.
Comparison of the reaction of different
boundaries, such as the
magnetopause and bow shock, at large distances to changes of
solar wind conditions is of a special
interest. Studies of
this kind can help answer some questions
on spatial/temporal interconnection between the boundaries occurring in the subsolar
region and distant magnetotail. They make it possible to evaluate the influence of
bow
shock processes
on the magnetopause location.

It is known that
the equilibrium position of the magnetopause is determined by
solar wind conditions: the dynamical pressure and
Bz component
of the interplanetary
magnetic field (IMF).
The magnetopause shape is described by a conical surface with
coefficients that depend on the solar wind condition
[Petrinec et al., 1996;
Shue et al.1997;
Sibeck et al.1991;
Roelof et al.1993].

The magnetospheric boundary moves according to the pressure balance between
the solar wind plasma and the Earth's magnetic field. Dynamical pressure
variations
in the magnetosheath can also result in magnetopause motion
[e.g.,
Nikolaeva et al., 1998].
The pressure decrease
related to the hot plasma fluxes flowing across the antisolar direction
(HFA stands for hot flow anomaly events) can lead to large amplitudes
(up to
5 RE )
of wave
motion of the magnetopause
[e.g.,
Sibeck et al., 1999].

The main task of this work is to
study the magnetospheric boundary
motion at large distances ( 30 RE ).
We have analyzed a few almost
simultaneous crossings of the
bow shock at the subsolar region and of the magnetopause
in the remote magnetotail. These boundaries were identified using plasma
and magnetic field data obtained onboard Interball 1 and Geotail satellites
[Klimov et al., 1997;
Kokubun et al., 1994;
Mukai et al., 1994;
Safrankova et al., 1997;
Sauvaud et al., 1997].
To determine interplanetary
magnetic field and solar wind conditions, the measurements at
Wind and partly at IMP 8
[e.g.,
Lepping et al., 1995;
Ogilvie et al., 1995]
were used.

2. Observations and Discussion

The magnetospheric boundary crossings
observed on 11 October 1996, are interesting
because the interaction of the solar wind
disturbance with magnetosphere near the subsolar region
and remote magnetotail took place almost simultaneously.

Figure 1

Figure 2

Figure 1 shows the spatial position of the
Interball 1,
Geotail, Wind, and IMP 8 satellites
during the 11 October event in the meridional (Figure 1a)
and equatorial (Figure 1b)
projections of the
geocentric solar ecliptic
(GSE)
coordinate
system. One can see from this figure that the Interball 1 satellite moved near
the
magnetopause predicted by the model on the low-latitude dusk flank
of the remote
magnetotail, Geotail crossed the subsolar bow shock region, and Wind and
IMP 8 were located within the solar wind.

Figure 2
presents one interval of the high-resolution (10 s)
plasma and magnetic field
measurements obtained by Interball 1
(the electron temperature
Te (eV), plasma
flux
Fi ( 108 cm
-2 s
-1 )),
and by Geotail (the magnetic field strength
|B| (nT),
ion temperature
Ti (eV),
longitudinal component of plasma velocity
Vx (km s
-1 ),
and plasma density
N (cm
-3 )).
The strip with a different degree of shadowing
(on top of the first and third panels) indicates
the regions passed by the satellites.
The regions for Interball 1 are as follows:
the magnetosheath (MSH), boundary layer (BL), and plasma sheet (PS).
For Geotail they are the solar wind (SW), magnetosheath (MSH), and
hot flow anomaly events (HFA). It should be noted that

Figure 3

the magnetic field and plasma velocity in Figures 2, 3, and 5
are presented in the Geocentric Solar-Magnetospheric (GSM) coordinates.

Figure 3
shows for the same time interval the solar
wind parameters (the magnitude
|B| and GSM components
of interplanetary magnetic field
Bx,
By,
Bz (nT),
dynamical pressure
Pd (nPa)) and also the
density
Np (cm-3),
and velocity
|V| (km s
-1 ) of the plasma measured by Wind
and partly by IMP 8.
The time delay caused by the solar wind
propagation from Wind to Interball 1
was taken into account. This value is about
T lag=28 min.
This time delay being taken into account, the magnetic field
data obtained by
IMP 8 and Wind became very similar.

One can see in Figure 3 the
passage of a large solar wind disturbance
region, edged by two irregularities
observed in the plasma and magnetic
field data at 0840 and 0920 UT, respectively. Both edges of this disturbance
region are characterized by
an abrupt magnetic field turning.
The dynamical pressure of the plasma within the region of
the solar wind disturbance
is decreased, and the interplanetary
magnetic field is northward. Lines I and III show the moments
of simultaneous BS and MP crossings.
Line II shows the moment of the first
HFA reaching Geotail. The moments I, II, and III are considered in more detail
below.

2.1. Boundary Crossing at 0840 UT

At this moment in response to the passage of solar wind
pressure depletion related
to the arrival of the leading front of the disturbance, almost simultaneous
movement (in the same direction:
from the Earth) of the outer boundaries was observed:
Interball 1
crossed the magnetopause at ~0840 UT
(entered the magnetosphere from the
magnetosheath, i.e.,
from the boundary layer), and Geotail passed the bow shock
(entered
the magnetosheath from the solar wind) 2 min
earlier.

Figure 4

Figures 4a and 4b
show the distance between the point of measurements and magnetopause
predicted by the
Shue et al. [1997]
empirical magnetopause model
(Figure 4b)
and the bow shock calculated from the Spreiter et al.
[1966]
hydrodynamic model
(Figure 4a).
Figure 3
shows the dynamical pressure and the
Bz (GSM)
component of the interplanetary magnetic field measured by Wind
which were used calculating
the model position of the magnetopause.

One can see in Figure 4b that the
predicted magnetopause was located at a larger distance
from the Earth than the measured one; the calculated
magnetospheric boundary expanded
outward responding to the variations of the solar wind parameters
(Figure 3).
One can assume that this degree of the relative boundary movement
( 0.8 RE away from the Earth) is sufficient to explain the
magnetopause crossing
observed at 0840 UT.

It
follows from Figure 4a that the bow
shock model prediction explains well the
observed bow shock crossing: before the arrival of the
disturbance leading front (0840 UT), Geotail
was located in the solar wind, and after interaction
with this irregularity, the satellite entered the magnetosheath.

The time delay between the boundary crossings, detected
by the two satellites separated
by a distance of
30 RE,
occurred 2 min instead of 10 min evaluated from the solar wind
propagation, if one assumes that the front of the
disturbance was plane and directed perpendicular to the
Sun-Earth line. This discrepancy in time can be
explained only by a large (38o)
inclination angle of the solar wind disturbance front and the
Sun-Earth line.
The components of the vector normal to the irregularity shows that the disturbance
front was inclined to the ecliptic plane by an angle of 62o.
Such an angle of the front inclination of the solar
wind disturbance helps in understanding the very
similar magnetic field behavior observed by the Wind and IMP 8
satellites, which were located
on the ecliptic plane and under it, respectively.

This orientation of the disturbance corresponds to the
results obtained by the statistic
analysis of inclination angles of solar wind disturbances
[e.g., Shukhtina et al., 1999].
It follows from the latter paper that
only a part (20%) of the solar wind disturbances
have the normal to an irregularity front
directed along the Sun-Earth line, and in the majority
of events (80%), they have large angles of the
front inclination about 30o-60o. Moreover,
a significant value of the vertical component of the
normal was often observed, and sometimes this component
was the main one.

Thus the observed magnetopause and bow shock positions are in a qualitative
agreement
with the model predictions. By varying solar wind parameters, one
can explain the observed almost simultaneous boundary crossings.

2.2. Bow Shock Crossing at 0920 UT

This bow shock crossing, observed by Geotail
at the moment II (0920 UT),
coincided with the arrival
of the trailing front of the solar wind disturbance.
At this time, Interball 1
was
still
located within the magnetosphere.
On its way from the magnetosheath into the solar wind, Geotail
crossed
several areas with unusual features (HFA), similar to the structures described, for
example, by
Paschmann et al. [1988],
Thomsen et al. [1993], and
Vaisberg et al. [1998].
These structures were filled with hot
tenuous plasma; the plasma moving fairly quickly transverse to the Sun-Earth
line and only slowly antisunward.

One can see in Figure 5
that the HFA event observed by Geotail at 0920 UT
has the following features:
a sharp rise of the ion temperature
Ti (up to 1000 eV), an abrupt decrease in the
Vx component of the plasma velocity and a significant
increase in the
Vy and
Vz components of the velocity,
a sharp decrease in the plasma
density; and a depletion of the magnetic field magnitude.
One can see
in Figure 5,
top panel,
the thermal pressure increase inside HFA (second event)
up to 1.5 nPa, the latter value being close to the dynamic pressure in MSH.
HFA-like regions
were observed by Geotail
during a long time interval of approximately 40 min (see Figure 2).

HFA events are characterized by a high temperature of ions (higher
than in the solar wind and
magnetosheath) and
by a
large transversed component of the plasma velocity
Vy=200-300 km s
-1,
whereas the
Sun-Earth
velocity is decreased down to
-50 km s
-1.
The magnetic field value within a HFA
is the same or slightly lower than in the solar wind. The
HFA events have a duration of about some minutes (up to 15 min)
and spatial dimensions of about several
RE. These events may
be intrinsic features of quasi-parallel
bow shocks, which in such a way react
to changes in solar wind conditions,
[e.g., Thomsen et al., 1993].

The formation of a hot flow anomaly near
the Earth's bow shock seems to be due to the
interaction between the bow shock and the impinging
irregularity in the upstream
plasma
[Thomsen et al., 1993].
Such an interaction will produce a HFA
if the electric field in the
ambient plasma is pointed toward the irregularity,
thereby focusing the shock-reflected ions into it. Assuming
that the irregularities are tangential, the predicted electric
field orientation is
found on the irregularity observed at 0920 UT.
It is essential that HFA are observed not only in
the solar wind but also within the
magnetosheath, that is, these cavities can pass
through the bow shock to the magnetosheath; and
move downstream. Its propagation toward
the magnetopause can induce changes
of the magnetopause shape
[e.g., Sibeck et al., 1999].

The top panel in Figure 5
presents a trace of the dynamic and thermal pressures associated
with the observed HFA events. One can use the pressure variations during
such events to predict
their effects on the
magnetosphere. One would expect the local magnetopause to expand rapidly
outward as the cavity passes and to contract rapidly inward
as the trailing edge of the first cavity passes (see the 0920-0922 UT interval
in
Figure 5).

The increase of the dynamic pressure at 0922 UT
and the exit of Geotail to solar wind
caused by it
(see
Figure 5,
top panel)
can produce the rapid magnetopause movement
inward which could be seen at Interball 1
5-7 min later (see its passage through the magnetopause
at 0925).
The pressure decrease within the third HFA event
can expand the magnetosphere as it is seen
at 0938 UT when Interball 1 entered the plasma sheet.
The plasma pressure increase observed by Geotail
when it entered the solar wind at 0930 UT
and the corresponding magnetosphere contraction can explain the
Interball 1 entry to the
magnetosheath observed at 0943-0948 UT (see Figure 2).
However, this comparison is very approximate, and
additional studies
of the influence
of the HFA events
on the boundary position in the magnetotail are needed.

2.3. Magnetospheric Boundary Crossings at 1000 UT

At this moment, Interball 1
left the magnetosphere and entered the
magnetosheath. Approximately 2 min earlier, Geotail
crossed the bow shock and
moved from the solar wind region into the magnetosheath (see Figure 2).
Thus we
observed an unusual situation: the dayside bow shock was expanded outside
the Earth, but the magnetotail
flank of the magnetopause was contracted inward.

In order for Interball 1, located inside the magnetosphere, to appear in the
magnetosheath, the flank boundary should have been shifted to the
Earth. At
the same time, in order for Geotail to
exit the solar wind and enter the magnetosheath,
the subsolar bow shock and probably the magnetopause should have been moved
away from
the Earth.

A similar magnetopause behavior
when the dayside boundary moves outward and
the distant magnetotail boundary moves to the Earth follows from the
magnetopause
models especially for the case of a very low dynamic pressure
[e.g., Roelof et al., 1993].
Then it is the interplanetary magnetic
field that provides the main influence on the
boundary location.
However, in our case, the
dynamic pressure was rather large
(up to 3 nPa), and the IMF
Bz component was changed only from
-2 to
-1 nT.

The other possible explanation includes an abrupt increase in the
magnetosheath
thickness in such
a way that the bow shocks were moved outside the Earth in spite of
the contraction of the entire magnetosphere. This situation is possible only
if the Mach number
in the solar wind was very low ( < 3). However,
in our case, the Mach number varied only
between 7 and 10.

One can see in Figure 4
that the observed variation of the solar wind parameters is not
sufficient to explain the boundary crossing observed by
Interball 1 at 1000 UT
(see Figure 4b).
At the same time, the bow shock model predictions are in a good
qualitative agreement with the boundary crossing observed by Geotail
(Figure 4a).

Thus the
large-scale boundaries motion observed from the subsolar region to the distant
magnetotail
(the dayside bow shock expansion and distant magnetopause contraction)
is poorly consistent with variations of the solar wind parameters.

An alternative explanation involves a boundary wave propagation along the
magnetopause. Since the HFA events are identified near the bow shock, it is
possible that the boundary wave could have been produced by hot flow anomalies.
A propagation of HFA events dawnward of the magnetotail can alter the local
magnetopause shape
[e.g., Sibeck et al., 1999].

3. Summary

1. Two cases with very
different types of boundaries motion have been observed during the 2-hour
interval on 11 October 1996:
the bow shock in
the dayside region of the magnetosphere and the magnetopause at the
magnetotail morning flank, the motions being detected by
the Geotail
and Interball 1 satellites,
which were separated by a distance of about
30 RE:
(1) in the first case,
the two boundaries (bow shock and magnetopause) almost
simultaneously moved outward due to the decrease of the solar
wind plasma dynamic pressure;
that is, the whole magnetosphere was expanded in
a qualitative agreement with the model predictions;
(2) in the second case, unusual boundaries behavior was observed:
almost
simultaneously the dayside bow shock was expanded from the Earth and the flank
magnetopause was contracted to the Earth.

2. The interaction between the solar wind
tangential irregularity and the subsolar
bow shock produced a whole set of the
HFA-like events characterized
by brief intervals of hot, relatively low density plasma with the bulk flow
strongly deflected from the antisolar direction.

3. We suggest that the observed unusual boundary
motions, which cannot be
explained by variations of solar wind parameters,
might be due to the background
of the HFA effects observed by Geotail before and after the bow shock
crossings. The plasma thermal pressure increase associated with HFA may
influence the magnetotail magnetopause position because the action of
the dynamic pressure in this region decreases less than the thermal pressure
found within HFA.

Acknowledgments

This work was partly supported by the Russian Foundation for
Basic Researches (grant 98-02-16297).